1. Field of the Invention
This invention relates generally to heat treatment apparatuses for processing semiconductor substrates and, more particularly, to thermal reactors and their temperature control systems.
2. Description of the Related Art
Reactors which can process a substrate while suspending the substrate without directly mechanically contacting the substrate, e.g., by floating the substrate on gas cushions, have relatively recently been developed for semiconductor processing. These reactors may be called floating substrate reactors. Such a reactor is commercially available under the trade name Levitor® from ASM International N.V. of Bilthoven, The Netherlands.
In the Levitor® reactor, which is also described in U.S. Pat. No. 6,183,565 B1, a substrate, such as a wafer, is supported by two opposite gas flows emanating from two heated and relatively massive reactor blocks located on opposite sides of the substrate. Each reactor block preferably also includes a heated section, or furnace body, for transferring heat to the substrate during processing. The furnace body preferably forms part of a boundary surface of the reactor block. The boundary surface is oriented to face the substrate and is preferably substantially flat. A small gap of less than about 1 mm is typically maintained between each block and the corresponding substrate surface. The small gap results in a rapid heat transfer from the furnace bodies to the substrate by conduction through the gas when the substrate is processed, e.g., during a heat treatment, or exposure to elevated temperatures. An advantage of reactors such as the Levitor® reactor is that the relatively massive reactor blocks of the reactor act as thermal “fly-wheels,” resulting in a very stable temperature and reproducible performance.
In addition, the heat-up of the substrate is very uniform, as the substrate is not mechanically contacted during the heat treatment. In comparison, where a transport arm transports a substrate into the reactor and then continues to support the substrate during processing, mechanical contact by support fingers of the transport arm can result in cold spots at their points of contact with the substrate during heat-up, as the support fingers represent extra thermal mass that must be heated and that locally slows down the heat-up rate during processing. Alternatively, where a substrate is transported to the reactor and then handed off to support pins that remain in the reactor after processing, mechanical contact by support pins during processing can result in hot spots on the substrate at their contact positions, as the support pins may already have been heated by a previous process. Advantageously, by floating a substrate during processing, these cold or hot spots can be avoided and thermal stresses, possibly resulting in crystallographic slip, can also be avoided.
Floating substrate reactors and methods for using such reactors for successive heat treatment of a series of planar substrates, one by one, is described in U.S. Patent Application Publication No. 2003/0027094 A1, published Feb. 6, 2003, and assigned to ASM International, N.V., the disclosure of which is incorporated herein by reference in its entirety. In those methods, a furnace body of a reactor block is typically continuously heated. After the furnace body has reached a desired temperature, as measured at a boundary surface of the furnace body, a relatively cold substrate is placed for heat treatment in the vicinity of the furnace body. Because it is generally colder than the furnace body, the substrate will withdraw heat from the furnace body. This heat withdrawal can be measured by measuring the temperature of the furnace body close to the boundary surface. The substrate is then heat treated. Because of the typically relatively short heat treatment time as compared to the thermal recovery time of the furnace body, the substrate is removed from the vicinity of the furnace body before the temperature of the continuously heated furnace body rises to the desired temperature again. After the temperature of the boundary surface rises to the desired temperature, another substrate is placed in the vicinity of the furnace body for heat treatment.
A purpose of the methods described in U.S. patent application Publication 2003/0027094 A1 is to achieve a reproducible heat treatment of each of the substrates. In the preferred embodiments of that application, this is achieved by having the furnace body at the desired temperature when a substrate is positioned for heat treatment, then removing the substrate from the vicinity of the furnace body before the temperature of the furnace body has recovered, and then waiting until the furnace body reaches the desired temperature again before positioning the next substrate in the vicinity of the furnace body for heat treatment. Thus, each substrate experiences roughly the same profile of heat treatment temperatures over time during the course of the heat treatment.
In addition, to make the temperatures over the boundary surface of the reactor block more uniform, the boundary surface typically comprises multiple heating zones, which can be controlled independently. By appropriately controlling the temperatures of these zones, a uniform temperature can be achieved over the part of the reactor block facing the substrate. Alternatively, the independent controls allow for a desired temperature gradient to be set up over the part of the reactor block facing the substrate.
Although the above-described methods and apparatuses allow for a reproducible thermal treatment of a series of substrates, the timing for treating successive substrates is relatively fixed, limiting the throughput flexibility of the reactor. In addition, even with independently controlled heating zones, the thermal recovery time may differ between the different heating zones, resulting in non-uniformities in the heat treatment temperature profile over the boundary surface.
Accordingly, there exists a need for apparatuses and methods of controlling the throughput of a floating substrate reactor and of making the temperature across a boundary surface of the reactor block more uniform.